The present invention relates to a wrapped cord comprising a core bundle of inorganic filaments, to a method of making said wrapped cord, to a method of treating said wrapped cord with a treatment composition and to the treated wrapped cord obtainable by said method as well as to a reinforced rubber article comprising said treated cord, such as automotive tires.
A tire is a highly engineered composite designed to provide safety and durability. Tires, in particular automotive tires for passenger cars or aircraft tires for aircrafts, undergo significant dynamic and static stresses and strains in the course of ordinary service life. Performance is critical in this product application due to ramifications of failure while in use. In order to obtain the necessary performance characteristics critical to the proper functioning of a tire, structural reinforcement is a required component of the tire composite. This reinforcement provides many functions in a tire application, in particular overall strength, dimensional stability for the tire and a mechanism to handle stress dissipation during operation (fatigue),
Currently, there is a well established set of products/processes to provide the reinforcing material used in passenger and truck tire applications.
1. High strength and ultra high modulus reinforcement materials like steel cord are used as belt in passenger and truck tires and as carcass in truck tires.
2. The steel cords contain steel filaments which are twisted to form a helix structured cord. The helix structure gives the cord bending flexibility and compressibility.
3. Each individual filament in steel cord is coated with brass layer. The brass layer gives a good adhesion between steel and rubber matrix. The adhesion is needed for stress transfer between cord and rubber matrix.
4. After a calendering process in which the steel cords are embedded in rubber, the rubberized cord layers can be introduced to the tire manufacturing process.
The step(s) that involve(s) twisting and plying is a critical operation in this series of processes. In this step, the proper construction and amount of twist must be established in order to obtain the proper fatigue resistance; however, this must be balanced against the loss in strength and modulus that occurs with twisting/plying as well as the costs for imparting twist, which increase with increasing twist levels. Much effort has been put into developing the proper twist levels to minimize cost and meet key durability requirements.
It has been shown that the twist imparted to the cord structure allows the cord to uniformly dissipate strain during compressive forces, the predominant forces (with respect to fatigue failure) that occur in service. The twist allows the cord to move out of plane during compression, thus avoiding catastrophic failure.
However, the conventional twisted cords suffer from modulus and breaking strength losses due to their helical constructions while having improved flex and compression fatigue resistance. The losses increase with increasing twist-level or helix-angle.
It has been an object of the present invention to provide a mechanism for strain dissipation and therefore fatigue resistance that does not require the conventional state-of-the-art twisting/cabling operations. In particular it has been an object of the present invention to provide a cord that combines the original yarn properties (a high breaking strength and, preferably, a high modulus) with an improved fatigue resistance.
Additionally, it has been an object of the present invention to provide a method for making said improved cord.
A further object of the present invention was to provide a such improved cord being treated with a treatment agent that promotes adhesion (adhesive agent) to rubber and said treated cord being ready to be introduced into the true manufacturing process where it is combined with rubber. Finally it has been an object of the present invention to provide a reinforced rubber article comprising the treated cord of the invention in the form of said cord itself or a fabric comprising said cord as a reinforcement.
It has been found that the above and further objects can be achieved by wrapping a low-denier, high shrinkage organic fiber (yarn) around a core bundle of inorganic filaments (yarns) resulting in a cord that resists fatigue while maintaining bundle coherency. The cord of the invention provides a mechanism for strain dissipation and therefore fatigue resistance, that does not require the twisting/cabling operations. The wrap material is wrapped in a helical pattern around the core, where wrap frequency and wrap angle can be specified based on performance requirements.
This cord structure according to the present invention has advantages over the conventional cord that is twisted and plied in that the elimination of twisting/cabling operations saves costs and, because the inorganic core ultra high modulus fiber is not twisted, there is no strength-loss of the core bundle in the cord. This allows fabric constructions to be modified to utilize less material to achieve the same strength and therefore reduces cost. Additionally, compared to conventional metal cords which are used in certain applications the cords according to the present invention help to reduce the weight of the reinforced article. In summary, the wrapped cord according to the present invention provides a cost-reduction for the formation of tire cord reinforcement and increased modulus while maintaining the necessary performance characteristics.